Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats

Correction for Inertia.


It is convenient to make one more scale, to show pressure per
square inch of piston. The piston area being 491 square ins.,
divide the total pressure reading by 491 to obtain reading per
sq. in.; stepped off at s/.

The indicator card for the top of the piston is set out by the
unit pressure scale at s/, and appears as EQXHB, the bottom of
diagram touching the base AB. Similarly FPGA is the card from
the bottom of the piston. Now, while QXHB is being drawn by
the indicator on top side of piston, A F R would be formed by
that connected with the bottom side, and the -effective pressure
will be the difference of these curve ordinates. Deduct those
at F from those at H, and the result is the curve WR. So also
VN is the curve of effective pressure on the bottom side of the

Now the actual total pressure to be carried forward to the
crank pin will be, during the first half of the stroke, less than that
on the indicator diagram by the amount required to set in motion
the reciprocating masses, viz., their inertia; and during the second
half of stroke the indicated pressure will be increased by the
backward pull needed to absorb inertia. Briefly, then, the c top'
card loses by the area ANS, and gains by SBP, the resulting
pressure area being NXWP; and similarly the resulting area for
the 'bottom' card will be P/VN. Setting up the resulting
ordinates on the straight base AB, we have the curve AŁ*/B for
the top and B <?/A for the bottom of piston, the total pressures
being written on each ordinate; and in order to equalise the
areas the cut-off in top diagram has been placed at '3 and in
bottom at '6 of stroke, the dead weights having to be supported
in the latter case.

We must next distinguish between reciprocating and rotating
parts, for only the former cause inertia force. The piston, piston
rod, crosshead, and smaller end of connecting rod are recipro-
cating weights, but the larger end of connecting rod is a rotating
weight As regards the connecting rod itself, about two-thirds
may be called reciprocating and the remaining third reckoned as
a rotative weight. The reciprocating weights directly affect the
indicator diagram, and the latter must be altered, by increased
compression or later cut-off, until a fairly even pressure is